Solvent recovery process

ABSTRACT

An improved process for the extraction of water soluble primary solvent contained in a raffinate hydrocarbon stream produced by a primary extraction process, wherein the primary solvent is contacted with an aqueous secondary solvent under extraction conditions in an extraction zone comprising an extraction tower containing a plurality of perforated extraction decks. The improvement comprises passing the solvent-containing raffinate through the perforations of the extraction decks at a hole velocity greater than 2 feet per second, and preferably in the range of from about 5 to about 8 feet per second. The process has particular application where the raffinate stream comprises nonaromatic hydrocarbons, the primary extraction process comprises an aromatic process, the secondary solvent comprises water, and the primary solvent comprises a sulfolane-type chemical solvent or a polyalkylene glycol solvent. One preferred application is in the recovery of sulfolane from nonaromatic raffinates in the water wash tower of an aromatic extraction processing plant.

United States Patent I 72] Inventor Herbert Lytle Thompson Park Ridge,111. [21] Appl. No. 835,544 [22] Filed June 23, 1969 [45] Patented Nov.9, 1971 [73] Assignee Universal Oil Products Company Des P1aines,lll.

[54] SOLVENT RECOVERY PROCESS 15 Claims, 3 Drawing Figs.

[52] US. Cl 208/321, 196/ 14.52 [51] 1nt.C1 Cl0g 21/28 [50] Field ofSearch 208/321; 196/ 14.52

[56] References Cited UNITED STATES PATENTS 2,614,031 10/1952 Tickler196/1452 2,667,407 1/1954 Fenske et a1 196/1452 2,767,068 10/1956Maycock et a1. 196/1452 2,921,015 1/1960 Shiras 208/321 RAFFINATE3,209,047 9/1965 Young Primary ExaminerHerbert Levine Att0rneys.lames R.Hoatson, Jr. and Philip T. Liggett ABSTRACT: An improved process for theextraction of water soluble primary solvent contained in a raffmatehydrocarbon stream produced by a primary extraction process, wherein theprimary solvent is contacted with an aqueous secondary solvent underextraction conditions in an extraction zone comprising an extractiontower containing a plurality of perforated extraction decks. Theimprovement comprises passing the solvent-containing raffinate throughthe perforations of the extraction decks at a hole velocity greater than2 feet per second, and preferably in the range of from about 5 to about8 feet per second. The process has particular application where theraffinate stream comprises nonaromatic hydrocarbons, the primaryextraction process comprises an aromatic process, the secondary solventcomprises water, and the primary solvent comprises a sulfolane-typechemical solvent or a polyalkylene glycol solvent. One preferredapplication is in the recovery of sulfolane from nonaromatic raffinatesin the water wash tower of an aromatic extraction processing plant.

NET r fi RAFFINATE 7 CLEAN WASH L. Y WATER l 1 l J l 3 I T l i.

| l l I l RICH WASH WATER PATENTEUNDV SHEET 1 OF 2 NET - RAFFINATE ZLEAN WASH T WATER (FEW i 1 J H- I L I 1 I I I '1 i 9 T 1 I3 '0 |4\ H 1 3T L C I RAFF|NAT'\ w l l l FIGURE l RICH WASH WATER I INVENTOR. HERBERTLYTLE THOMPSON A 7' TOR/V575 SOLVENT RECOVERY PROCESS FIELD OF THEINVENTION The present invention relates to the solvent extraction ofaromatic hydrocarbons from a hydrocarbon charge stream. Moreparticularly, the present invention relates to the recovery of solventfrom the nonaromatic raffmate produced by the solvent extraction ofaromatics form a hydrocarbon charge stream. Most specifically, thepresent invention relates to an improved process for the recovery ofsolvent from the nonaromatic raffmate by means of a secondary extractionprocess.

It is well known in the art that the nonaromatic raftinate which leavesthe extraction zone of an aromatic hydrocarbon extraction processcontains solvent. The solvent which is withdrawn in the raffimate streammust be recovered not only because it may interfere with subsequentraffmate processing or ultimate raffinate use, but primarily becausecontinual loss of solvent in the raffinate stream is a prohibitiveeconomic expense in the aromatic extraction process. The recovery of thesolvent from the raffmate stream may be accomplished by distillation, oradsorption, or by a secondary solvent extraction process.

A typical solvent which is utilized in commercial aromatics extractionand which may be recovered in accordance with the practice of thisinvention is a solvent of the sulfolane type. The solvent possesses afive-membered ring containing one atom of sulfur and four atoms ofcarbon, with two oxygen atoms bonded to the sulfur atom of the ring.Generically, the sulfolane type solvents may be indicated as having thefollowing structural formula:

wherein R,, R R and R are independently selected from the groupcomprising a hydrogen atom, and alkyl group having from one to tencarbon atoms, and alkoxy radical having from one to eight carbon atoms,and an arylalkyl radical having from one to 12 carbon atoms.

Other solvents which may be included within this process are thesulfolenes such as 2-sulfolene or 3-sulfolene which have the followingstructures:

2-Sulfolene Other typical solvents which have a high selectivity forseparating aromatics from non-aromatic hydrocarbons and which may beprocessed within the scope of the present invention are2-methylsulfolane, 2,4-dimethylsulfolane, methyl 2- sulfonyl ether,n-aryl-3-sulfonyl amine, 2-sulfonyl acetate, diethylene glycol, variouspolyethylene glycols, dipropylene glycol, various polypropylene glycols,dimethyl sulfoxide, N- methyl pyrollidone, etc.

The specifically preferred solvent chemical which is processed withinthe scope of the present invention is sulfolane, having the followingstructural formula:

DESCRIPTION OF THE PRIOR ART Because the typical solvents which areutilized in aromatics extraction are water soluble, it is the practiceto extract the solvent from the raftinate stream by contact with anaqueous stream in a subsequent extraction means. The extraction of thesolvent from the raffinate with water may be undertaken in any suitableliquid-liquid contacting means, as in a tower containing suitablepacking such as Berl Saddles or Raschig Rings, or in a tower containingsuitable trays, or in a rotating disc contactor (RDC). The solvent maythen be readily recovered from the aqueous solution by distillation.

One typical extraction tower containing extraction trays, wherein thesolvent is recovered by extraction from the nonaromatic raftinate withwater, is the so-called "rain deck" extraction tower. In this type ofoperation, the extraction tower contains a plurality of perforatedextraction trays or decks containing upcomer conduit means. The waterphase collects above the deck and passes down through the perforationscontained therein, while the hydrocarbon phase is contained under thedeck and passes up to the next contacting stage via the upcomer conduitmeans contained in the deck. The water falling through the perforationsof the extraction deck rains down through the hydrocarbon phase which isimmediately under the deck. Thus the name rain deck" is typicallyutilized for this type of water wash tower.

Another version wherein a suitable tray device is used in water washingthe nonaromatic rafi'mate in order to recover the solvent therefrom,comprises a plurality of perforated decks wherein downcomer conduitmeans are provided. In this type of deck, the water is retained abovethe trays and passes down to the contacting stage below via thedowncomer conduits. The hydrocarbon which is contained below the deckpasses up through the perforations of the deck and up through the waterphase thereby providing for extraction of the solvent contained in theraffinate by the water phase which is on the deck.

IN utilizing this type of water wash extraction tray or deck, it iscommon in the art to provide a 2 foot spacing between adjacent trays ofthe plurality. It is further the practice to provide that the spacebetween the decks contain about I foot of liquid level of aqueous phaseabove the deck, and about 1 foot of hydrocarbon phase above the aqueousphase. In addition, it is the practice to pass the hydrocarbon upthrough the perforations in the decks at flow rates below a holevelocity of 2 ft./sec., and preferably to operate with a hole velocityof about 1 ft./sec. These limitations of tray spacing and hole velocityare imposed in such water wash operations in order to minimizeentrainment of the phases, and maximize the extraction efficiency forthe decks.

It has been discovered, however, that in commercial aromatic extractionunits the recovery of solvent from the raffinate by extraction with thewater does not correspond to the recovery which is to be anticipatedbased upon solubility data and the assumption of the reasonableefficiency of the extractor.

It is obvious in the art to provide additional physical stages in theaqueous extractor in order to enhance the recovery of the solvent. Sucha solution to the problem of poor extraction efficiency is technicallyfeasible but it is not a preferred solution since it requires that thenumber of physical stages in the aqueous extractor must be increased.Not only in this a prohibitively uneconomical expedient but once acommercial unit has been placed on stream it is often a physicalimpossibility to modify the existing facility to provide the requiredadditional contacting stages. The preferred solution to the problem,therefore, is to subject the solvent rich raffmate stream to extractionconditions which will render the raffinate means.

SUMMARY OF THE INVENTION It is, therefore, an object of the presentinvention to provide a process for the recovery of water soluble solventfrom a nonaromatic rafiinate stream by aqueous extraction.

It is a particular object of the present invention to provide a meansfor the recovery of water soluble solvent from a nonaromatic raffinatestream in an aqueous extraction means containing a minimum number ofphysical stages.

it is a more specific object of the present invention to minimize thenumber of physical stages in the aqueous extraction means andsimultaneously achieve recovery of water soluble solvent in a morefacile and economical manner.

It has been determined that these objectives may be achieved by bringingthe nonaromatic raffinate stream into contact with an aqueous phase inan extraction zone containing not a greater number of physical stages,but containing a fewer number of physical stages than have heretoforebeen utilized in the art.

it has now been determined that increased efficiency of extraction maybe obtained and a fewer number of physical stages may be utilized withinthe extraction zone, if the nonaromatic raffinate is passed upwardthrough the perforations of the extraction trays at hole velocitieswhich are substantially greater than what has been heretofore utilizedin the prior art. As noted hereinabove, in extraction operations whereinthe raffinate is water washed by being passed up through theperforations of the extraction trays, while the water phase passes tothe tray below via downcomer means, the raffinate has been passedthrough the perforations of the extraction trays at hole velocitiesconsistently below 2 feet per second, and more normally at holevelocities of about 1 foot per second. It has now been determined thatby the practice of the present invention, increased efficiency ofextraction is obtained at hole velocities substantially greater than 2ft./sec., and in particular that increased economic benefits can beachieved with hole velocities in the range of from about 5 ft./sec. toabout 8 ft./sec.

Therefore, in accordance with the practice of the present invention, oneembodiment comprises an improved process for the extraction of watersoluble primary solvent contained in a raffinate stream produced by aprimary extraction process, wherein the primary solvent is contactedwith an aqueous secondary solvent under extraction conditions in anextraction zone comprising an extraction tower containing a plurality ofperforated extraction decks, wherein the improvement comprises passingthe solvent containing raffinate stream through the perforations of theextraction decks at a hole velocity greater than 2 ft./sec.

As shall be set forth more clearly hereinafter, the present inventionmay be further characterized as a process for separating a water solubleprimary solvent from a solvent containing raffinate hydrocarbon streamproduced in a primary extraction process which comprises, contacting theraffinate hydrocarbon stream in a contacting zone with a hereinafterspecified hydrocarbon stream under conditions sufficient to provide amixed hydrocarbon stream containing the water.

soluble solvent; passing the mixed hydrocarbon stream into an extractionzone comprising an extraction tower containing a plurality of perforatedextraction decks, wherein the mixed hydrocarbon stream is contacted witha first stream of aqueous secondary solvent under extraction conditions,said conditions comprising the flow of the mixed hydrocarbon through theperforations of the extraction decks at a hole velocity greater than 2ft./sec.; withdrawing from the extraction zone a second stream ofaqueous secondary solvent containing the primary solvent; and,withdrawing from the extraction zone a mixed hydrocarbon stream havingsubstantial freedom from the primary solvent, and passing a portionthereof to contacting zone as the specified hydrocarbon stream.

The present invention may be further characterized by the twoembodiments set forth immediately hereinabove wherein the raffinatestream comprises nonaromatic hydrocarbons, the

primary extraction process comprises an aromatic extraction process, thesecondary solvent comprises water, the primary solvent comprises eithera sulfolane type chemical or at least one polyalkylene glycol.

The process of the present invention and its operational basis, isclearly set forth in the accompanying figures. FIG. 1 consists of asimplified schematic flow diagram illustrating the broad embodiment ofthe invention and one preferred process wherein the invention ispracticed. FIG. 2 comprises a set of curves obtained form laboratorydata wherein there is shown the enhanced extraction of solvent from thenonaromatic raffinate within increased hole velocities as the raffinatepasses through the perforations of the extraction tray. FIG. 3illustrates design data for a specific example of raffinate compositionand shows the influence of hole velocity upon different designconsiderations for the extraction tower.

The inventive process may now be more readily understood by discussingthe figures in greater detail.

FIGURE I As noted hereinabove, FIG. 1 sets forth a simplified schematicflow diagram of the process wherein the present invention may bepracticed.

Referring now to FIG. 1, a typical nonaromatic raffinate streamcontaining a primary solvent in solution, leaves an aromatic extractionzone, not shown, at a temperature normally in excess of 150 F. Thisraffinate stream is cooled in heat exchanger means, not shown, andenters the process of the present invention via line 1 at a temperatureof F. or less. The raffinate stream passes via line 1 to an aqueousextraction or water wash tower 2 which contains a plurality ofperforated extraction trays 3. Each extraction tray comprises aperforated deck and suitable downcomer means. Since those skilled in theextraction art are familiar with extraction tray construction, theperforated trays 3 are not shown in great detail and, for illustrativepurposes only, are shown as single pass extraction trays.

Water Wash Tower 2 is maintained under operating conditions sufficientto remove the primary solvent from the nonaromatic raffinate by means ofa lean wash water which passes into the water wash tower via line 4.Among these operating conditions are a temperature in the range of from60 to F., and normally a temperature of about 100 F. The pressure mustbe sufficient to keep all constituents in a liquid state, and thepressure will, therefore, be normally in excess of 30 p.s.i.g. The leanwash water is injected into the water wash tower 2 at a mol ratio ofwater to raffinate hydrocarbon, in the range of from 0.5 mols per mol to2.0 mols per mol. Normally, when sulfolane is the primary solvent, thewash water passes into the extraction tower 2 via line 4 at a ratesufficient to maintain from 1 mol to 1.4 mols of water for every mol ofnonaromatic raffinate entering the tower. When a polyalkylene glycolsolvent is the primary solvent to be recovered from the raffinate, thenormal range of operation comprises 0.5 mols to 1.0 mols of water forevery mol of raffinate containing the glycol solvent.

The extraction trays within the water wash tower 2 may be any type ofsuitable extraction tray, provided that they are operated with theraffinate passing up through the perforations in the decks. This meansthat each deck will support a layer of aqueous phase which is maintainedthereon and overflows via downcomer means to the extraction tray below.The hydrocarbon phase is confined below each extraction tray and abovethe aqueous phase which is held on the extraction tray below. As therafiinate passes up through the tower, it passes through theperforations of each extraction tray 3 at a hole velocity greater than 2fi./sec., and preferably at a hole velocity in the range of from about 5to about 8 ft./sec.

By operating the water wash tower 2 under the conditions set forthhereinabove, an improved extraction efficiency is obtained and theprimary solvent is removed from the raffinate with increased recovery. Arich wash water leaves the bottom of the raffinate water wash tower 2via line 5 and passes to subsequent processing for the recovery of theprimary solvent therefrom. The net raffinate leaves the water wash tower2 via line 6, and passes either to subsequent processing or to storagefacilities, not shown. The net raffinate will contain a lower quantityof primary solvent than has been heretofore experienced in the prior artmanner of operation of hole velocities below 2 ft./sec.

It is common in the art of aromatic extraction to design the aromaticextraction plant with the capability of running at 50 percent of designflows. Consequently, provision must be made to maintain the operation ofthe water wash tower 2 at the desired hole velocities in the extractiontray in order to continue to obtain increased efficiencies of extractiondespite fluctuations in raffinate production. In order to achieve this,a portion of the net raffinate may be recycled so that the holevelocities in the extraction trays will be maintained at the desiredrates.

REferring again to FIG. 1, when the raffinate entering the process vialine 1 decreases in rate of flow, either through a decrease in the rateof charge of hydrocarbon to the aromatic extraction tower, not shown, ordue to change in the composition of the feed to the aromatic extractiontower, the raffinate rate to water wash tower 2 will be reduced. Undersuch circumstances, a portion of the net raffinate is withdrawn fromline 6 via line 7 at a controlled rate. The rate of flow via line 7 iscontrolled by flow control valve 8 in a manner sufficient to maintainthe total flow of hydrocarbon through the water wash tower 2 at ratessufficient to produce the desired hole velocity through the perforationsin the trays 3. The recycled portion of the net raffinate passes throughthe control valve 8 and into line 9. It hereafter passes through a blockvalve 10 and into the water wash tower 2 via line 11. This is thepreferred method of return for the net raffinate which is recycled tothe water wash tower 2. The fresh raffinate containing the primarysolvent is contacted with the recycle raffinate under the lowest of theextraction trays 3, and the mixed hydrocarbon stream then passes upthrough the perforations of the extraction tray at the desired holevelocity.

As an alternative method, the portion of the raffinate which is recycledin order to maintain the hole velocity within the water wash tower 2,may be withdrawn via line 12 and passed through block valve 13 beforepassing into line 1 via line 14. When such an operation is utilized, theblock valve 10 in line 9 remains closed and the-contacting of the freshraffinate containing the primary solvent, with the recycle raffinatewhich is substantially free of solvent, occurs in line 1 before themixed raffinate stream passes into the water wash tower 2.

Those skilled in the art will thus perceive that the contacting zonewherein the solvent-containing feed raffinate is contacted with therecycled portion of solvent-free rafi'inate, may be either internal orexternal to the water wash tower which contains the aqueous extractionzone.

FIGURE II As indicated hereinabove, FIG. 2 sets forth an example ofexperimental laboratory data which shows that increased hole velocityproduces an increased extraction efiiciency in accordance with thepresent invention.

The data shown in FIG. 2 were obtained from laboratory experiments whichwere made for a single extraction deck utilizing a nonaromatic raffinatefrom a commercial extraction plant. The raffinate was blended withvarious concentrations of primary solvent and synthetic wash water wasmade by blending pure water with various concentrations of primarysolvent. Various concentrations of primary solvent in the wash waterwere made in order to simulate operation at various locations (upper andlower) of a typical water wash tower.

The raffinate which was utilized in the series of experiments had agravity of 69.8 API at 60 F., and it had a boiling range of 155 F. to357 F. This raffinate comprised 98.2 liquid volume percent ofnonaromatics and L8 liquid volume percent of aromatics. The raffinatewas blended with various amounts of primary solvent and utilized in allthe experimental work.

Referring now to FIG. 2, there is shown a plot of the percent extractedvs. hole velocity. Three curves are containing in FIG. 2 and all curvesare for an extraction deck containing circular perforations 0.2083centimeters in diameter. Curve A shows the increased efficiency which isobtained by increasing hole velocity when contacting the raffinatecontaining 50 ppm. of chemical sulfolane with a wash water containingppm. of the sulfolane solvent. Curve B is a similar curve for contactingthe raffinate containing 550 ppm. sulfolane with water containing 150ppm. sulfolane. Curve C illustrates the contacting of raffinatecontaining 550 ppm. of sulfolane with water containing 10,000 ppm. ofsulfolane.

The curves of FIG. 2 clearly show that increased hole velocity increasesextraction efficiency regardless of the amount of solvent contained inthe hydrocarbon phase and regardless of the amount of solvent containedin the aqueous phase. Similar curves were obtained on other extractiontests utilizing other hole sizes.

FIGURE In FIG. 3 sets forth curves for illustrating maximumeffectiveness of the present invention as applied to a commercial waterwash tower design. In FIG. 3, design considerations are set forth forwater washing a primary solvent comprising chemical sulfolane from araffinate having he same characteristics as the raffinate disclosed inthe discussion relative to FIG. 2. That is to say, the design basiscomprises a raffinate having a gravity of 69.8 API, containing 1.8liquid volume percent of aromatics, and having a boiling range of fromF. to 357 F.

The design apparatus basis for the curve contained in FIG. 3 is a 4-footdiameter extraction vessel containing extraction decks having %-inchdiameter holes. Those skilled in the art realize, of course, the holesize is not of specific significance since the hole velocity is thecritical factor in the present invention. The design basis for thiswater wash tower is a hole velocity at 75 percent of flooding velocity.The design basis, further, is for an extractor tray containingdowncomers sufficient to hold I foot of aqueous phase on each deck.

The design operational basis is for an aqueous extraction on theraffinate to reduce the raffinate content from 550 ppm. of sulfolanesolvent in the input raffinate, to produce a net raffinate hydrocarboncontaining only 1.0 ppm. of sulfolane.

Utilizing the data obtained in the laboratory for sulfolane solventsystems, a curve is obtained which relates the number of extractiontrays required within the water wash tower vs. the hole velocity throughthe extraction trays in feet per second. This curve illustrates that asthe hole velocity increases the total number of trays required isdecreased for a specific recovery of sulfolane solvent.

The next curve which is illustrated in FIG. 3 is a plot of the trayspacing within the water wash tower vs. the hole velocity through theextraction deck in feet per second. Those skilled in the art realizethat tray spacing is a calculated figure obtained by the hydraulicbalance for the specific extractor deck being utilized. More simply put,the tray spacing is in effect the amount of liquid head of watercontained within the downcomer which is needed to drive the hydrocarbonphase up through the holes of the extraction tray, and through 1 foot ofwater phase contained on the deck above, at a given hole velocity.Referring to FIG. 3 it is seen that as hole velocity increases the trayspacing required between adjacent extraction trays increasesaccordingly.

The next curve contained within FIG. 3 is a plot of column height vs.hole velocity. This curve is based upon the knowledge of the number oftrays utilized at a given hole velocity and the tray spacing requiredfor that hole velocity. This curve of column height, however, is notproportional to these two factors since in addition to the calculatedcolumn height due to number of trays and tray spacing, there is provideda 14 foot additional tower height for the bottom and the top of thewater wash tower. This additional distance of 14 feet is provided inorder to provide a settling zone at the top of the water wash tower anda similar zone at the bottom of the water wash tower. These zonesprovide a quiescent zone at the top and the bottom wherein thehydrocarbon and aqueous phases may be settled out before being withdrawnfrom the water wash tower. It is seen from this curve in FIG. 3, that asthe hole velocity increases and tray spacing thereby increases, thecolumn height additionally increases to an apparent maximum at a holevelocity of about 9 ft./sec. Above this hole velocity, so few extractiontrays are'required in the water wash tower that it is readily apparentwhy the column height decreases.

The three curves of FIG. 3 define operation at a maximum extractionefficiency. That is to say, the design basis provided for aqueousextraction of a nonaromatic raffinate containing 550 p.p.m. of chemicalsulfolane in a water wash tower to produce a raft'inate containing only1.0 p.p.m. of sulfolane. Thus the extraction efficiency was in excess of99 percent.

The next question, therefore, is where this maximum efficiency can beobtained at lowest expense. The cost of a water wash tower isessentially equal to the cost of the column and the cost of theextraction trays contained therein. Cost estimates of the water washtower were made based upon the curves of FIG. 3. It was found that athole velocities in excess of 5 ft./sec. the cost of the water wash towerwas substantially equivalent. However, it was also found that atvelocities below 5 ft./sec. the cost of the water wash tower increasedwith decreasing hole velocity. At low hole velocities, the column wasshorter but a greater number of extraction trays were needed to obtainthe constant degree of extraction. Those skilled in the art realize thatextraction trays are an expensive item of equipment. Thus, having ashorter water wash tower containing more extraction trays, is moreexpensive than having a taller tower containing a fewer number ofextraction trays positioned at a larger tray spacing.

It may, therefore, be concluded from the from the data that reducedcapital expense is obtained by maintaining the extraction hole velocityin excess of 2 ft./sec. and preferably in excess of 5 ft./sec.

From the curves contained within FIG. 3 it might be inferred thatmaximum economy and maximum efficiency would be obtained at a higherhole velocity. However, other considerations limit the preferredoperation to a maximum hole velocity of about 8 ft./sec.

As noted hereinabove, tray spacing is fixed by hydraulic balance for agiven hole velocity. Higher hole velocities require a higher trayspacing up to about 8 ft./sec. It was found in the laboratory, that therequired tray spacing was adequate for settling of phases so thatentrainment was not so great that the extraction efficiency benefitsderived by high bole velocity was lost. Thus, it was found that a holevelocity of up to 8 ft./sec. and a tray spacing of up to 12 feet couldbe utilized to obtain a water wash tower at minimum cost while stillobtaining the benefit of improved extraction efficiency. The laboratorydata indicated that at hole velocities exceeding 8 ft./sec., entrainmentbecame a problem such that there is the danger that under some operatingconditions, the tray spacing demanded for proper hydraulic balance maynot be sufficient to allow proper settling out of the phases between thetrays. Therefore, the efficiency of extraction derived by high velocitycould be lost due to entrainment problems. While additional tray spacingcould be added to obtain proper settling of phases, the added spacingresults in a tower height such that the capital cost may becomeprohibitive.

By referring again to FIG. 3, another reason may be found for limitingthe operation to a maximum hole velocity of 8 ft./sec. In FIG. 3, itwill be seen that the number of trays required for the extraction at 8ftJsec. is only four, and that at hole velocities above 8 ft./sec. areduced number of extraction trays are required. By limiting the waterwash tower to such a small number of extraction trays, there isintroduced into the water wash extraction step, the danger of anineffective water washing of the primary solvent from the nonaromaticraffinate. This danger is that under some circumstances of operation,the aromatic extraction unit wherein the water wash tower comprises oneoperating step, may be subjected to flow fluctuation and operationalupsets. If such an upset should occur and the water wash tower containsless than four extraction trays, a substantial slut" of the primarysolvent may enter the water wash tower with the nonaromatic raftinateand pass out of the tower without being extracted out of the raffinate.It is, therefore, desirable to provide a sufficient number of extractiontrays within the water wash tower to minimize the danger that any upsetor operational swings at the.aromatic extraction column will produceupsets or flow fluctuations at the water wash tower sufficient to allowthe loss of a substantial amount of the primary solvent. Thus, in thepreferred embodiment the water wash tower should contain at least fourextraction trays and preferably more than four, so that hole velocitiesin excess of 8 ft./sec. must be avoided.

In conclusion, therefore, it may be summarized that the effectiveness ofthe present invention is enhanced by operating the solvent recovery stepwith the raffinate passing through the perforations of the extractiontrays contained within the water wash tower at hole velocities in excessof 2 ft./sec. and preferably with hole velocities within the range offrom 5 to 8 ft./sec. The effectiveness of this preferred range ofoperation may be illustrated by the following example which is basedupon the operations of a commercial aromatic extraction unit.

EXAMPLE A commercial aromatics extraction unit was operated on anaromatics concentrate feedstock to extract benzene, toluene, xylene andC +ar0matics form the hydrocarbon feed. A net raffinate was withdrawnfrom the aromatic extraction process which had a boiling range in theC,,C range and a gravity of 62.9 API. This raftinate stock contained 2.7mol percent of aromatic hydrocarbons and 1.2 mol percent sulfolanesolvent.

Referring now to FIG. 1, the raftinate entered the water wash tower ofthe commercial aromatic extraction unit via line 1 at a rate of 2263b.p.s.d. (barrels per stream day) or a rate of 212.3 mols per hour. Atthese flow rates, the rafiinate feed to the water wash tower 2 contained2.5 mols per hour of sulfolane solvent which was required to berecovered in the commercial process.

The water wash tower 2 contained seven extraction trays 3 having 54-inchholes. The tray spacing within the extraction tower was 92 inchesbetween adjacent extraction trays (7 ft. 8 inches A wash water streamentered the water wash tower 2 via line 4 at a rate of 371 b.p.s.d. or arate of 300 mols per hour. This lean wash water was derived from theextract stripping column of the commercial aromatic extraction unit, andthe wash water therefore contained about I00 p.p.m. of sulfolane solventand a trace of low boiling aromatic hydrocarbon.

The raffinate stream entered the extractor column 2 via line 1 andpassed upward through the perforations of the seven extraction trays ata hole velocity of about 6.1 ft./sec. The water wash operation wasmaintained at a temperature of about I00 F. and a pressure of 65p.s.i.g.

A net wash water was withdrawn from the bottom of the water wash tower 2via line 5 at a rate of 387 b.p.s.d. or 302.5 mols per hour. This netwash water contained 2.5 mols per hour of sulfolane solvent which wasextracted from the hydrocarbon phase in the extraction zone comprisingthe seven extraction trays.

The net raffinate product was withdrawn from the top of the water washtower 2 via line 6 at a rate of 2247 b.p.s.d. or 209.8 mols per hour.The net raffinate product from the water wash tower consistentlyanalyzed a sulfolane solvent content in the range of from about 10p.p.m. of sulfolane to about 12 p.p.m. of sulfolane.

amount of sulfolane solvent. Prior art experience indicates that typicaloperation would require water washing of the raffinate at holevelocities less than 2 ft./sec., and normally 1 ft./sec., through awater wash tower containing a substantially greater number of extractiontrays than was utilized in the commercial operation described herein,Under such prior art operating conditions, it is anticipated that thenet raffinate product recovered via line 6 would contain from about 50ppm. of sulfolane solvent to about 100 ppm. sulfolane solvent.

During the commercial operation described hereinabove, no recycle ,ofraffinate was required since sufficient raffinate product was producedat the aromatics extraction column to maintain the hole velocitiesrequired within the water wash tower. However, during other .periods ofthe commercial operation, charge rates to the aromatic extraction towerwere cut back and recycle of the net raffnate wasrequired'at the waterwash tower. The recycle of the raffinate was conducted via line 7,control valve 8, line 9, line 12, block valve 13, and line 14, therebyeffecting contact of the recycle-raffinate and the fresh raffinate feedexternally to the water wash tower. Under suchcircumstances of operationit was found that since the hole velocity was maintained in range offrom 6.0 to 6.4 ft./sec. at all times, the extraction of the primarysulfolane solvent from the rafi'rnate phase by the wash water continuedto be maximized sothat the net raffinate product contained substantiallythe same low quantity of sulfolane solvent as was experienced when norecycle of raffinate was required.

PREFERRED EMBODIMENT The effectivenessv of the present invention and theprocess of the present invention have been clearly set forth by thedisclosure hereinabove.

It is particularly noteworthy that the above disclosure sets forth amethod whereby increased extraction efficiencyis effected whilesimultaneously reducing the number of physical stages required within,the extraction zone. Therefore, the process of the present inventionsets forth a method whereby an existing aqueous extraction or water washtower may be modified to improve extraction of the primary solvent fromthe raffinate, without requiring the insertion of additional masstransfer stages. Contrary to what the prior art. would lead one toexpect, improved extraction efficiency may be obtained by removingextraction trays from the existing aqueous extraction tower instead ofadding trays. Of course, the operation must be further modified toprovide for the passage of raffinate through the perforations of theextraction trays at greater hole velocities, but this is simply providedby installing a process line for-the recycleof a portion of the netproduct raffinate in the manner disclosed hereinabove.

Thus, the present invention provides increased recovery of primarysolvent at minimum expense for future water wash systems to beconstructed, and for existing water wash systems which must be modifiedto correctproblems of poor solvent recovery.

Those skilled in the art realize that the effectiveness of the presentinvention is influenced by a great many factors; For example, the degreeof extraction depends upon the specific primary solvent which is beingremoved from the raffinate hydrocarbon stream by the water wash step..In addition, the effectiveness and the operating conditions requiredwill be influenced by the temperature of the raffinate stream enteringthe water wash tower, the solvent content of the raffinate streamentering the,inventive process from the aromatic extraction unit, thesolvent content of the lean wash water, and the specific operatingconditionswithin the water wash tower. It must further be noted that.the solvent content of the nonaromatic raffinate will vary, since it isdependent upon the temperature level of the preceding aromaticextraction processing unit and upon the mo] percent'of aromaticsremaining in the nonaromatic raffinate stream. lt is, therefore, notpossible to define specifically the operating conditions which arerequired within the water wash tower. However,

those skilled in the art can readily ascertain the operating con- Iditions which may be required for any specific raffinate composition byutilizing the teachings which have been presented hereinabove.

Therefore, it may now be summarized that a preferred embodiment of thepresent invention comprises an improved process for the extraction ofwater soluble primary solvent contained in a raffinate stream producedby a primary extraction process, wherein the primary solventis'contacted with an aqueous secondary solvent under extractionconditions in an extraction zone comprising an extraction towercontaining a plurality of perforated extraction decks, wherein theimprovement comprises passing the solvent containing rafi'rnate streamthrough the perforations of the extraction decks at a hole velocity inthe range of from about 5' ft./sec. to about 8 it may further besummarized, that another preferred embodiment of the 'present:inventioncomprises a process for separating a water soluble primary solvent froma solvent containing raffinate hydrocarbon stream produced in a primaryvsoluble solvent; passing the mixed hydrocarbon stream into anextraction zone comprising an extraction tower containing a plurality ofperforated extraction decks wherein the mixed hydrocarbon stream iscontacted with a first stream of aqueous secondarysolvent underextraction conditions, the conditions comprising the flow of mixedhydrocarbon through the perforation 'ofthe extraction deck at a holdvelocity in the range of from about 5 ft./sec. to'about 8 ft./sec.;withdrawing from the extraction zone a second stream of aqueoussecondary solvent containing the primary solvent; and withdrawing fromthe extraction zone a-mixed hydrocarbon stream having substantialfreedom from the primary solvent, and passing a portion thereof to thecontacting zone as the specified hydrocarbon stream.

The invention claimed:

1. In a process for the extraction of water soluble primary solventcontained in a nonaromatic hydrocarbon raffinate stream produced by aprimary aromatic extraction process,

. wherein said primary solvent is contacted with an aqueous wherein RR,,-R and R are independently selected from the group comprising ahydrogen atom,an alkyl group having from one to ten carbon atoms, anarylalkyl radical having from one to 12 carbon atoms, and an alkoxyradical having from one to eight carbon atoms.

4. Process of claim 3 wherein said primary solvent comprises sulfolane.

5. Process of claim 2 wherein said secondary solvent comprises water,and said primary solvent comprises a sulfolene selected from the groupconsisting of Z-sulfolene and 3-sulfolene.

6. Process of claim 2 wherein said secondary solvent comprises water,and said primary solvent comprises at least one polyalkylene glycol.

7. Process of claim 6 wherein said primary solvent comprises at leastone glycol selected from the group consisting of diethylene glycol,dipropylene glycol, and triethylene glycol.

8. Process for separating a water soluble primary solvent from asolvent-containing nonaromatic raffinate hydrocarbon stream produced ina primary aromatic extraction process which comprises:

a. contacting said raffinate hydrocarbon stream in a contacting zonewith a hereinafter specified hydrocarbon stream under conditionssufficient to provide a mixed hydrocarbon stream containing said watersoluble solvent;

b. passing said mixed hydrocarbon stream into an extraction zonecomprising an extraction tower containing a plurality of stationaryperforated extraction decks, wherein said mixed hydrocarbon stream iscontacted with a first stream of aqueous secondary solvent underextraction conditions,said conditions comprising the flow of mixedhydrocarbon through the perforations of said extraction decks at a holevelocity greater than 2 feet per second;

c. withdrawing from said extraction zone a second stream of aqueoussecondary solvent containing said primary solvent; and,

d. withdrawing from said extraction zone a mixed hydrocarbon streamhaving substantial freedom from said primary solvent, and passing aportion thereof to said contacting zone as said specified hydrocarbonstream.

9. Process of claim 8 wherein said hole velocity is maintained in therange of from about 5 ft./sec. to about 8 ft./sec.

10. Process of claim 9 wherein said secondary solvent comprises water,and said primary solvent comprises a sulfolanetype chemical of thegeneral formula:

wherein R R R and R are independently selected from the group comprisinga hydrogen atom, an alkyl group having from one to ten carbon atoms, anarylalkyl radical having from one to 12 carbon atoms, and an alkoxyradical having from one to eight carbon atoms.

11. Process of claim 10 wherein said primary solvent comprisessulfolane.

12. Process of claim 9 wherein said secondary solvent comprises water,and said primary solvent comprises a sulfolene selected from the groupconsisting of 2-sulfolene and 3-sulfolene.

13. Process of claim 9 wherein said secondary solvent comprises water,and said primary solvent comprises at least one polyalkylene glycol.

14. Process of claim 13 wherein said primary solvent comprises at leastone glycol selected from the group consisting of diethylene glycol,dipropylene glycol, and triethylene glycol.

15. Process of claim 8 wherein said contacting zone comprises a lowersection of said extraction tower and said extraction zone comprises anupper section of said extraction tower containing at least a portion ofsaid plurality of extraction decks.

2. Process of claim 1 wherein said improvement comprises a hole velocityin the range of from about 5 ft./sec. to about 8 ft./sec.
 3. Process ofclaim 2 wherein said secondary solvent comprises water, and said primarysolvent comprises a sulfolane-type chemical of the general formula: 4.Process of claim 3 wherein said primary solvent comprises sulfolane. 5.Process of claim 2 wherein said secondary solvent comprises water, andsaid primary solvent comprises a sulfolene selected from the groupconsisting of 2-sulfolene and 3-sulfolene.
 6. Process of claim 2 whereinsaid secondary solvent comprises water, and said primary solventcomprises at least one polyalkylene glycol.
 7. Process of claim 6wherein said primary solvent comprises at least one glycol selected fromthe group consisting of diethylene glycol, dipropylene glycol, andtriethylene glycol.
 8. Process for separating a water soluble primarysolvent from a solvent-containing nonaromatic raffinate hydrocarbonstream produced in a primary aromatic extraction process whichcomprises: a. contacting said raffinate hydrocarbon stream in acontacting zone with a hereinafter specified hydrocarbon stream underconditions sufficient to provide a mixed hydrocarbon stream containingsaid water soluble solvent; b. passing said mixed hydrocarbon streaminto an extraction zone comprising an extraction tower containing aplurality of stationary perforated extraction decks, wherein said mixedhydrocarbon stream is contacted with a first stream of aqueous secondarysolvent under extraction conditions, said conditions comprising the flowof mixed hydrocarbon through the perforations of said extraction decksat a hole velocity greater than 2 feet per second; c. withdrawing fromsaid extraction zone a second stream of aqueous secondary solventcontaining said primary solvent; and, d. withdrawing from saidextraction zone a mixed hydrocarbon stream having substantial freedomfrom said primary solvent, and passing a portion thereof to saidcontacting zone as said specified hydrocarbon stream.
 9. Process ofclaim 8 wherein said hole velocity is maintained in the range of fromabout 5 ft./sec. to about 8 ft./sec.
 10. Process of claim 9 wherein saidsecondary solvent comprises water, and said primary solvent comprises asulfolane-type chemical of the general formula:
 11. Process of claim 10wherein said primary solvent comprises sulfolane.
 12. Process of claim 9wherein said secondary solvent comprises water, and said primary solventcomprises a sulfolene selected from the group consisting of 2-sulfoleneand 3-sulfolene.
 13. Process of claim 9 wherein said secondary solventcomprises water, and said primary solvent comprises at least onepolyalkylene glycol.
 14. Process of claim 13 wherein said primarysolvent comprises at least one glycol selected from the group consistingof diethylene glycol, dipropylene glycol, and triethylene glycol. 15.Process of claim 8 wherein said contacting zone comprises a lowersection of said extraction tower and said extraction zone comprises anupper section of said extraction tower containing at least a portion ofsaid plurality of extraction decks.